I. Field of the Invention
The present invention relates generally to hydraulically-powered, multiple rotor, riding trowels with various rotor diameters, and hydraulic control circuits used in such trowels. More particularly, the present invention relates to a riding trowel using hydraulic circuitry including an unloader circuit responsive to hydraulic feedback for critically regulating the pump output flow to operate within the engine horsepower envelope. Riding trowels of this general type are classified in United States Patent Class 404, Subclass 112.
II. Description of the Prior Art
High power, multiple rotor, hydraulic riding trowels for finishing concrete are well recognized by those skilled in the art. Proper finishing insures that desired surface characteristics including appropriate smoothness and flatness are achieved. It is also important that delamination be minimized. High power, hydraulically driven riding trowels are capable of finishing large areas of plastic concrete quickly and efficiently, while insuring high quality surface characteristics.
Modern hydraulic power riding trowels comprise two or more bladed rotors that project downwardly and frictionally contact the concrete surface. In advanced machines the rotors are driven hydraulically from hydraulic drive motors pressured by hydraulic pumps that are in turn powered by a separate, internal combustion engine. The riding trowel operator sits on top of the frame and controls trowel movement with a joystick steering system that tilts the rotors for control. The weight of the trowel and the operator is transmitted frictionally to the concrete by the revolving blades or pans. Frictional forces caused by rotor tilting enable the trowel to be steered.
Holz, in U.S. Pat. No. 4,046,484 shows a pioneer, twin rotor, self propelled riding trowel. U.S. Pat. No. 3,936,212, also issued to Holz, shows a three rotor riding trowel powered by a single motor. Although the designs depicted in the latter two Holz patents were pioneers in the riding trowel arts, the devices were difficult to steer and control.
Prior U.S. Pat. No. 5,108,220 owned by Allen Engineering Corporation, the same assignee as in this case, relates to a manual steering system for riding trowels. Mechanical gearboxes were used for rotor propulsion.
Allen Engineering Corporation Pat. No. 5,613,801 issued Mar. 25, 1997 discloses a power riding trowel equipped with twin motors. The latter design employs a separate motor to power each rotor. Steering is accomplished with structure similar to that depicted in U.S. Pat. No. 5,108,220 previously discussed.
Older manually operated trowels used hand levers to develop rotor tilting movements for steering. Rotors were driven by internal combustion motors transmitting force through rotor gear boxes. Manually operated systems with gearbox-driven rotors have been largely replaced with hydraulic trowels. For example, U.S. Pat. No. 5,890,833 entitled “Hydraulically controlled riding trowel” issued to Allen Engineering Corporation Apr. 6, 1999 discloses a high performance, hydraulic riding trowel using a joystick system that controls steering, propulsion, and blade pitch. A rigid trowel frame mounts two or more downwardly-projecting, bladed rotor assemblies that frictionally engage the concrete surface. The rotor assemblies are tilted with double acting hydraulic cylinders to effectuate steering and control. Double acting hydraulic cylinders also control blade pitch. The joystick system activates solenoid control valves that energize various hydraulic cylinders that tilt the rotors and alter blade pitch.
U.S. Pat. No. 6,089,786 entitled “Dual rotor riding trowel with proportional electro-Hydraulic Steering” issued Jul. 18, 2000 and U.S. Pat. No. 6,053,660 issued Apr. 25, 2000 and entitled “Hydraulically controlled twin rotor riding trowel” disclose joystick-operated, twin rotor riding trowels for finishing concrete. The trowel frame mounts two spaced-apart, downwardly projecting, and bladed rotors that frictionally contact the concrete surface. The rotors are tilted with double acting, hydraulic cylinders for steering and control. Double acting hydraulic cylinders also control blade pitch. A joystick system enables the operator to hand control the trowel with minimal physical exertion. The joystick system directly controls electrical circuitry that outputs proportional control signals to electrically control the steering or tilting cylinders. The hydraulic circuitry comprises a motor driven pump delivering pressure to a flow divider circuit.
U.S. Pat. No. 6,048,130 issued Apr. 11, 2000 and entitled “Hydraulically driven, multiple rotor riding trowel” and U.S. Pat. No. 5,816,739 entitled “High performance triple rotor riding trowel” disclose related, triple rotor hydraulic trowels.
U.S. Pat. No. 6,106,193 entitled “Hydraulically driven, Multiple Rotor riding trowel issued Aug. 22, 2000 discloses high performance, hydraulic riding trowels for finishing concrete. Separate hydraulic motors revolve each rotor assembly. Associated hydraulic circuitry engenders convenient joystick control.
U.S. Pat. No. 6,857,815 entitled “Acoustic impedance matched concrete finishing” issued Feb. 22, 2005 discloses a method for matching the acoustic impedance of concrete treating equipment to the acoustic impedance of the concrete slab being treated. A twin rotor riding trowel is provided with a pair of circular finishing pans that are attached to the conventional rotor blades. The pans are characterized by acoustic impedance approximating the acoustic impedance of plastic concrete, thereby optimizing the energy transferred to the concrete. The matching material comprises ultra high molecular weight polyethylene (UHMWPE) plastic. During troweling, the pans are frictionally revolved over the plastic concrete for finishing the surface without prematurely sealing the uppermost slab surface, to produce a highly stable concrete surface with minimal delamination.
U.S. Pat. No. 7,108,449 entitled “Method and apparatus for acoustically matched slip form Concrete Application” issued Sep. 19, 2006 involves the concept of acoustic matching discussed in Allen U.S. Pat. No. 6,857,815 and employs it with slip form pavers.
U.S. Pat. No. 7,114,876 entitled “Acoustically matched concrete finishing pans” issued Oct. 3, 2006 to Allen Engineering Corporation discloses improved acoustically matched pans for riding trowels. The pans are provided with means for matching the acoustic impedance of the concrete slab being treated as discussed in Allen U.S. Pat. No. 6,857,815.
German Pat. No. G9,418,169.1 entitled “Concrete smoothing machine” issued Jan. 26, 1995 to Betontechnik Shumacher GmbH discloses a riding trowel.
U.S. Pat. No. 5,816,740 entitled “Hydraulically controlled steering for power trowel” issued Oct. 6, 1998 to Timothy S. Jaszkowiak discloses dual-acting hydraulic cylinders interconnected to the rotors and the frame for steering.
U.S. Pat. No. 6,048,130 entitled “Hydraulically driven, multiple rotor riding trowel” issued Apr. 11, 2000 to Allen Engineering Corporation discloses a hydraulically-propelled multiple rotor riding trowel utilizing hydraulic motors and circuitry.
U.S. Pat. No. 2,869,442 entitled “Floating and troweling machine” issued Nov. 29, 1956 to John M. Mincher discloses a floating and troweling machine for finishing plastic floors which is constructed so that it can controlled by an operator seated on the machine.
U.S. Pat. No. 4,320,986 entitled “Motor powered rotary trowel” issued Mar. 23, 1982 to Donald R. Morrison discloses a trowel with radially arranged trowel blades which can be adjustably tilted on their support arms in either direction and are mounted on a drive shaft which can be driven in either direction.
U.S. Pat. No. 4,676,691 entitled “Dual rotary trowel” issued Jun. 30, 1987 to Donald R. Morrison discloses a concrete troweling machine having two sets of troweling blades with a mechanism for setting the tilt of individual blades in a rotor assembly.
U.S. Pat. No. 4,977,928 entitled “Load sensing hydraulic system” issued Dec. 18, 1990 to Caterpillar Inc. discloses a hydraulic load sensing system and more particularly a hydraulic system in which one of the pressure compensated flow control valves is rendered inoperative during certain operating conditions of the system.
Barikell located in Australia has two versions of a hydraulic controlled riding trowel. The “MK8-120 HCS” and the “OL-120 HCS Overlapper” are the trowels noted. (http://www.barikell.com.au/)
Tremix located in Sweden has a hydraulic controlled riding trowel called the “Pro Rider” in which the machine is controlled by two joysticks that act directly upon the guiding valves. There are two foot pedals, one adjusting the revolutions of the engine, the other opening/closing the valves to the hydraulic engines. (http://www.tremix.com/eng/concrete/prorider.html)
An article found on an internet web page entitled “Insider secrets to Hydraulics” reveals how to understand hydraulic load sensing control in control circuits. (http://www.insidersecretstohydraulics.com/hydraulic-load-sensing.html)
MBW Inc. whose headquarters are in Slinger, Wis. U.S.A. has a riding trowel called the “MK8 121” in which the machine is controlled by two hydraulic joysticks.
Multiquip Inc. whose headquarters are in Carson, Calif. U.S. A. has two riding trowels that are hydraulically controlled and driven. The STX-55Y-6 and the HTX-44K-5 models are detailed in a MQ-WRPT-1797 Rev. H (01-08) brochure entitled “Ride-on Power Trowels”.
An article in a January 2005 issue of Concrete Construction Magazine written by Ted Worthington states a riding trowel called the Tarantula. It is manufactured by a company called Full-Track BVBA located in Belgium. (http://www.concreteconstruction.net/industry-news.asp?sectionID=707&articleID=566833)
Bosch Rexroth Corporation has a product they manufacture entitled the “Power Valve” and is used to control a variable displacement pump's operating pressure. This item is detailed in a September 1999 brochure RE 95 514/09.99 distributed by Rexroth.
Bondioli and Pavesi Inc. has a product they manufacture entitled the “Power limiter control valve” and is used to maintain maximum power from a power source by sensing operating pressure of the hydraulic circuit. This item is detailed in a quick reference hydraulic catalog provided by Bondioli and Pavesi entitled “QH008”.
Sauer Danfoss has a product they manufacture entitled the “MCV106A Hydraulic Displacement Control (HDC)” and is uses mechanical feedback to establish closed-loop control of the swashplate angle of various pumps provided by Sauer Danfoss. This control is explained in article BLN-95-8972-3 issued March 1991 by Sauer Danfoss.
Notwithstanding numerous attempts at maximizing the speed of troweling, along with the pursuit of high quality concrete finishes, new problems have developed in the art.
Speed increases in surface finishing have made it possible for larger quantities of concrete to be placed in a given job environment in a given time. Modem placement speeds exceed the speed at which concrete was placed several years ago. Contractors routinely expect to finish thousands of square feet of surface area after placement. Panning and troweling stages commence when the concrete is still plastic.
Concrete undergoes numerous well recognized changes in its physical chemistry between the initial mixing stages and the final hardening stages. For example, as diagrammed in FIG. 1 below, in the initial mixing stage, high heat is generated followed by rapid cooling. This initial stage lasts about fifteen minutes, and it is critical that the mixture be adequately mixed. During the ensuing dormancy period, which lasts about two to four hours, the concrete mixture is plastic and workable, and high heat is no longer generated. At the beginning of the dormancy period, the plastic concrete is typically confined within a delivery vehicle during transportation to the job site. After transportation, delivery, and placement, various diverse finishing techniques follow. As concrete is laid, it can be struck off for initial shaping. Typically, screeding follows. At this time significant moisture may rise to the surface.
The subsequent hardening or hydration stage, which generates significant heat, lasts about two to four hours. The mixture sets, begins to harden, and the slab gains strength. Panning ideally starts at the “initial set” point indicated in FIG. 1, which is approximately between the dormancy and cooling stages. Large, circular metal pans are temporarily secured to the trowel rotors for panning. Alternatively, plastic pans, or acoustically matched pans, can be used. As the concrete hardens, pans are removed and blade troweling finishes the job. Often, multiple trowels, equipped with different pans or blades, are employed in stages.
After panning, when the concrete has gotten harder, blade troweling follows. Vigorous blade troweling continues through the hardening period. In the following cooling stage, stresses are developed within the slab, and stress relief, typically relieved by sawing, is required.
However, in typical construction, as large areas of concrete are poured and finished, wet, freshly poured concrete regions will often border harder regions. Large riding trowels rapidly traverse large areas of fresh concrete surface, and it is not uncommon for their spaced-apart rotors to simultaneously contact surface regions of varying hardness and frictional characteristics. Severe, potentially damaging stresses on the trowel drive train can result.
Further, when a trowel enters a plastic region of wet concrete characterized by a high friction, as can happen when panning stages encounter wet concrete too early, the severe power drain significantly slows the internal combustion engine powering the trowel. The same thing can happen when a trowel encounters concrete that is too plastic during blade troweling of a large, curing slab. When the rotors are overloaded, even if momentarily, engine droop can occur, stalling follows, and normal engine output drops. Internal combustion engines are particularly vulnerable to stalling and power drops in such circumstances. With hydraulic trowels, this sudden power drop reduces the hydraulic operating pressure below optimum levels, affecting trowel steering and control. Sometimes the sudden fluctuation in operating pressure, particularly if the engine stalls completely, can result in surface damage to the concrete from irregular rotor movements.
As a practical matter, stalling can occur when the required horsepower from the engine in a given situation exceeds the maximum horsepower available. Normally with hydraulic riding trowels it is desirable to maintain drive engine RPM within a relatively limited range at a favorable operating point. Sudden demands placed on the engine by the hydraulic system can place too much demand on the drive engine. Such condition causes reduced engine life, degraded trowel performance, overheating, and a reduction of finish quality. The horsepower required is a function of rpm and rotor torque. To optimize trowel operation, as rotor torque increases, rotor rpm can then be reduced to promote operation within the desired engine horsepower limits. When rotor load conditions occur where maximum rotor torque and maximum rotor rpm are required simultaneously, the corresponding engine horsepower availability may be inadequate.
In a typical hydraulic riding trowel an internal combustion engine drives one or more hydraulic pumps. The typical hydrostatic piston pump in a twin-rotor trowel drives two hydraulic rotor motors. A mechanical stroking device, including a mechanical arm that pivots a swash plate, can increase or decrease rotor rpm. Two mechanical arms connected by a common linkage are linked to a foot pedal controlled by an operator. When the foot pedal is depressed, the linkage creates a turning torque to the swash plates on both pumps. Resulting increased pump displacement creates increased flow to turn the rotor motors at an increased rpm. The stroking mechanism forces are dictated by piston pump pressure. As pressure increases, the holding torque needed to maintain position increases. This is a natural condition for a direct, mechanically operated stroking operation for a piston pump swash plate. To maintain swash plate position, and therefore rotor speed, the trowel operator takes corrective action by pushing harder on the foot-pedal. A rider's instinctive action is to further push the trowel foot pedal, which can stall the engine, with the consequences, discussed above.
Thus a solution is required to prevent riding trowel internal combustion engines from overloading and over-stressing in response to diverse RPM and torque requirements encountered upon varying concrete surfaces.
In using hydraulically driven riding trowels in the field, a problem with internal combustion engine overload was discovered. Severe overloading stresses the hydraulic components. One way to overcome the overloading problem is to increase pressure in the hydraulic system. The latter approach results in two problems however: not enough torque to the rotors, and the machine could not perform at higher engine RPM and torque without stalling the engine. The torque required to turn the rotors is directly proportional to the weight of the machine. By using the operating parameters of the hydraulic riding trowel, the torque needs to finish the concrete can be measured. As the torque needs are less, the frictional forces are less, and can be measured in coefficient of friction values. During the window of finishability, two times of peak load occur, each during the pan and blade operations. At one point during the panning operation the surface is such that the coefficient of friction is larger than usual. At this time the invention is very useful to moderate this condition of heavy loading. Similarly during the blade operation this peak occurs at a point of finishing the concrete. During the process of finishing the concrete there is a typical amount of loading. Only at these moments of peak loading is there a spike in the demand of horsepower. This condition is somewhat unpredictable due to the different mixture content of the concrete and environmental conditions. Only in very large pours with rapid concrete placement can this be observed with any regularity. Most times this is elusive to observe in a small pour. It does occur in spots, however, and this will be very detrimental to efficient work using a smaller powered ride on trowel. Space and weight limitations prevented using higher horsepower engines. There was a need to increase torque and reduce weight so a redesign of the machine was required. A partial solution is to increase the displacement on the rotor motors, which increases torque and reduces RPM. It was concluded that the torque envelope required for proper operation sacrifices rotor RPM and internal combustion drive engine RPM. A solution could not be achieved with the existing system. The goal was to provide a system that could not be burdened by the operator and which would optimize performance levels of torque and rotor RPM without engine overload.
A system was designed that would allow the control of flow from the pump to the rotor motors based upon operating pressure. This would control the total horsepower required by the machine. When the set torque limit was obtained, rotor RPM was reduced to stay within the available engine horsepower. In a light load situation, there would be low torque and high RPM. In a heavy load situation, there would be high torque and lower RPM.
Thus it is proposed to monitor hydraulic system conditions, and to derive a corrective hydraulic feedback signal. A responsive unloading valve system is proposed to decrease rotor RPM at a maximum preselected torque limit and to increase rotor speed at a minimum predetermined torque limit. Simultaneously, it is important that the internal combustion engine operate within the optimum engine horsepower curve. Engine stalling is reduced, if not avoided altogether, notwithstanding the continually fluctuating surface frictional characteristics as depicted by the hydration curve (FIG. 1) of the concrete regions being traversed by the trowel.